**Core Concept**
The patient's presentation of fever and pain in the upper right hypochondrium after food intake suggests acute cholecystitis, which is likely caused by gallstones blocking the cystic duct. This condition requires prompt diagnosis and treatment to prevent complications.

**Why the Correct Answer is Right**
The investigation of choice for acute cholecystitis is ultrasound (US), which can accurately diagnose gallstones and assess gallbladder inflammation. US is preferred due to its non-invasive nature, lack of radiation, and high sensitivity for gallstones. The patient's symptoms of pain after food intake, known as biliary colic, further support the diagnosis of gallstones.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is not the first-line investigation for acute cholecystitis, although it may be used in complex cases or when ultrasound is inconclusive.

**Option B:** MRI is not typically used for the initial evaluation of acute cholecystitis due to its higher cost and limited availability compared to ultrasound.

**Option C:** ERCP (Endoscopic Retrograde Cholangiopancreatography) is a more invasive procedure used for therapeutic purposes, such as removing stones or stenting the bile duct, rather than initial diagnosis.

**Clinical Pearl / High-Yield Fact**
Ultrasound is the preferred initial investigation for acute cholecystitis due to its accuracy, non-invasiveness, and lack of radiation, making it an ideal choice for patients with suspected gallstone disease.

**Correct Answer: A. Ultrasound.